BASHIQA, Iraq (REUTERS) - Iraqi Kurdish peshmerga forces attacked an Islamic State in Iraq and Syria (ISIS)-held town north-east of Mosul on Monday (Nov 7), trying to clear a pocket of militants outside the city while Iraqi troops wage a fierce urban war with the Islamists in its eastern neighbourhoods.

As the campaign against ISIS' Iraqi stronghold entered its fourth week, fighters across the border launched an offensive in the Syrian half of the terror group's self-declared caliphate, targeting its base in the city of Raqqa.

The assault on Raqqa, held by ISIS for nearly three years, will be spearheaded by armed groups backed by the United States and supported by US-led air strikes. Unlike in Iraq, where the army is leading the assault, however, it not being coordinated with President Bashar al-Assad or the Syrian army.

In Bashiqa, some 15km from Mosul, the first waves of a 2,000-strong peshmerga force entered the town on foot and in armoured vehicles or Humvees.

Artillery earlier pounded the town, which lies on the Nineveh plains at the foot of a mountain.

"Our aim is to take control and clear out all the Daesh (ISIS) militants," Lieutenant Colonel Safeen Rasoul told Reuters. "Our estimates are there are about 100 still left and 10 suicide cars."

ISIS fighters have sought to slow the offensive on their Mosul stronghold with waves of suicide car bomb attacks. Iraqi commanders say there have been 100 on the eastern front and 140 in the south.

A top Kurdish official told Reuters on Sunday (Nov 6) the terrorists had also deployed drones strapped with explosives, long-range artillery shells filled with chlorine gas and mustard gas and trained snipers.

As a peshmerga column moved into Bashiqa on Monday, a loud explosion rocked the convoy, and two large plumes of white smoke could be seen just 15 metres away. A peshmerga officer said two suicide car bombs had tried to hit the advancing force.

"They are surrounded... If they want to surrender, OK. If they don't, they will be killed," said Lieutenant Colonel Qandeel Mahmoud, standing next to a Humvee, supported by a cane he said he has needed since he was wounded in the leg by two suicide car bombers four months ago.

In eastern districts of Mosul, which Iraqi special forces broke into last week, officers say Islamists melted into the population, ambushing and isolating troops in what the special forces spokesman called the world's "toughest urban warfare".

Mosul, the largest ISIS-controlled city in either Iraq or Syria, has been held by the group since its fighters drove the army out of northern Iraq in June 2014.

The Mosul campaign, the most complex military operation in Iraq in a decade, brings together a force of around 100,000 soldiers, security forces, Shi'ite militias and peshmerga, backed by a US-led coalition, to crush the Sunni terrorists.

Across the border, US-backed Syrian fighters have launched their own campaign, called Euphrates Anger, to recapture Raqqa.

Twin offensives on Raqqa and Mosul could bring to an end the self-styled caliphate declared by ISIS leader Abu Bakr al-Baghdadi from the pulpit of a Mosul mosque in 2014.

Baghdadi, whose whereabouts are unknown but who is believed to be in northern Iraq close to the Syrian border, has told his followers there can be no retreat in a "total war" with their enemies.

The militants in Mosul have been waging a fierce and brutal defence, although they have lost ground on all fronts outside the city itself.

To the south of Mosul, security forces said they had recaptured and secured the town of Hammam al-Alil from ISIS fighters, who they said had kept thousands of residents as human shields as well as marching many others alongside retreating militants towards Mosul as cover from air strikes.

The security forces on the southern front have continued their advance, reaching within 4km of Mosul's airport, on the southern edge of the city and on the western bank of the Tigris River which runs through its centre.

To the north, a military statement said the army's Sixteenth Infantry Division had also recaptured the village of Bawiza and entered another area, Sada, on the city's northern limits, further tightening the circle of forces around ISIS.

Shi'ite militias known as Popular Mobilisation forces are also fighting to the west of Mosul to seal the routes to the ISIS-held town of Tal Afar and its territory in neighbouring Syria, to prevent any retreat or reinforcement.

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